The invention concerns a printed valuable document with at least one authentication feature in the form of a luminescent substance based on a host lattice doped with at least one rare earth metal.
The protection of valuable documents by means of luminescent substances has long been known. The use of rare earth elements in this connection has also been discussed. These have the advantage of possessing narrow band emission lines that are particularly characteristic and can therefore be safely distinguished from the emissions of other substances when using measuring technology. Preferably substances are used that have emission lines in the invisible region of the spectrum, especially in the infrared (IR) spectral region.
In order to enhance protection against counterfeiting, the rare earth metals can be incorporated with other substances into host lattices in such a manner that the excitation or emission spectrum of the rare earth metal is influenced in a characteristic manner. Through combination with suitably absorptive substances, for example, a part of the excitation and/or emission bands of the rare earth metal can be suppressed. This influence, however, may also take the form of a xe2x80x9cdistortionxe2x80x9d, for instance through damping of particular areas of the excitation or emission spectra.
Starting from this state of the art, the invention is based on the aim of producing a valuable document with an authentication feature in the form of luminescent substances that, compared with the state of the art, are more difficult to detect and therefore offer greater security against counterfeiting.
The fulfilment of this aim is embodied in the non-dependent claims. Further developments of these are the subject of the dependent claims.
As already stated, for the purposes of checking the authenticity of valuable documents, the emission lines of rare earth metals lying in the IR spectral region are used. Preferably, emission lines are used that lie in the near IR, since these can still be detected with low cost sensors and, because of the favourable signal-to-noise ratio, erroneous measurements can be avoided relatively easily. Normally, commercially available silicon (Si) or germanium (Ge) detectors are used for this. The further into the IR spectral region the emission lines lie, the more difficult it is to detect the emissions. The reason for this is that, in general, the detection sensitivity or response sensitivity of photodetectors decreases the longer the wavelength of the radiation to be measured. This means that the signal-to-noise ratio of the detected signals generally becomes smaller with increasing wavelength. As a result, the measuring technology needed to evaluate the signals and the necessary expertise becomes ever more expensive. If these luminescent substances that are technically difficult to detect are also only present in the valuable documents under test in small concentrations, detection of the emission lines is only possible under special conditions.
The invention is based on the recognition that this increasingly difficult detectability of certain substances with increasing emission wavelength in the IR region can be employed to great advantage to increase the level of counterfeit protection.
According to the invention, therefore, in order to protect valuable documents, a luminescent substance is used whose emission spectrum lies beyond the response sensitivity of Si or Ge detectors, or at least at the border of detectability for a Ge detector. In this case, the technical effort required for detection with a Ge detector must be multiplied many times or, for instance, lead sulphide (PbS), indium arsenide (InAs), gallium-indium arsenide (GaInAs) or lead selenide (PbSe) detectors used. However, these detectors"" detection sensitivity is powers of ten lower than that of Si detectors, so that the technical evaluation of the signals from a detector of this kind is always significantly more difficult.
The substances suitable for the authenticity protection could be materials based on holmium-doped or thulium-doped host lattices. Thulium produces emission lines in the wavelength range from 1.6 to 2.1 xcexcm, and especially in the range 1.7 to 1.9 xcexcm, and holmium produces emission lines in the range from 1.8 to 2.1 xcexcm. The emission spectra of the two rare earth doping-metals therefore overlap each other, so that with suitable stimulation of both doping substances, the emission intensity is increased in the overlap area. By this means, the emission signal stands out strongly from the background noise. The double doping also has the advantage that, on the basis of the emission spectrum, conclusions can be drawn concerning the optically active elements of the luminescence material. Furthermore, in comparison to the use of just one rare earth metal, there are more emission lines available for evaluation. In this way, it is more difficult for potential counterfeiters to find out which of the lines are actually evaluated during authenticity checking.
Thulium can just be detected with Ge detectors with correspondingly high levels of technical effort, since the response sensitivity of Ge detectors at a wavelength of 1.6 xcexcm is already very small, while at 1.9 xcexcm, it tends towards zero. Holmium, on the other hand, cannot be detected with Ge detectors. The emissions of both rare earth metals can, however, be detected with PbS, InAs or GaInAs detectors. Since the response sensitivity of these detectors is also very low in the wavelength range from 1.7 to about 2.1 xcexcm, the thulium and the holmium must be incorporated into a host lattice that ensures the highest possible effectiveness of the doping substancesxe2x80x94that is, one that provides for the highest possible quantum yield. According to the invention, host lattices are used that contain broad-band absorptive components and transfer the absorbed energy highly efficiently to the rare earth doping metals. Preferably, the quantum yield of the luminescent substances according to the invention lies in the range between 50 and 90%.
In addition, the invention provides for use of the luminescent substance in the relevant valuable document at such a concentration that just avoids impairing the characteristics of the valuable document. The maximum concentration depends on various parameters, such as, for instance, the manner of application or the desired characteristics (colour or similar) of the valuable document.
If, for instance, the luminescent substance is embedded in a paper pulp, then the maximum allowable concentration of foreign substances is just a few percent by weight. If the allowable foreign substance concentration is exceeded, this results in marked changes to the material properties. For instance, too high a foreign substance concentration in the paper reduces the tear resistance of the paper. If the luminescent substance has its own colour, then a concentration of only about 0.1 percent by weight may suffice to change the colour of the entire paper. An excessively high foreign substance concentration in printing inks makes the inks brittle and reduces their adhesion on the document surface. In this case, also, a concentration of just 1 percent by weight of a coloured luminescent substance my suffice to distort the entire colour appearance of the printing ink. If, on the other hand, this luminescent substance serves simultaneously as a colour pigment, the limit concentration may only be reached at the physical maximum of possible solid substance content of about 80 percent by weight.
According to the invention, the lower limit concentration in the case of colourless or slightly coloured luminescent substances for mixing into the paper pulp is around 0.1 percent by weight. In the case of more strongly coloured luminescent substances, the limit concentration may lie at just 0.01 percent by weight. Preferably, the concentration is in the range from 0.05 to 1 percent by weight. The lower concentration limit of the luminescent substance in a layer applied to the valuable document, however, is about 1 percent by weight, for example in the case of coloured luminescent substances. Depending on the layer composition and application purpose, the concentration is in the range from 1 to 40 percent by weight, and preferably in the range from 10 to 30 percent by weight.
The incorporation of luminescent substances at a concentration that is just admissible i.e. one that does not impair the paper""s named characteristicsxe2x80x94prevents counterfeiting attempts where, in ignorance of the real luminescent substances, less effective replacement materials with similar emission lines are used, but which must be added to the valuable document in higher concentrations in order to produce measurable signals. This leads to recognisable changes in the valuable document or to the printing ink containing the luminescent substance. In the case of coloured substances, for instance, this would lead to a discoloration of the valuable document or the printing ink.
According to the invention, the luminescent substances can be applied to the valuable document in a variety of ways. For example, luminescent substances can, as already mentioned, be added to a printing ink, which also contains visually detectable pigments. The luminous materials can also be added to the paper pulp. The luminescent substances can also be provided on or in a plastic base material which, for instance, is at least partially embedded into the paper pulp. The base material may take the form of a safety thread, a mottling thread or a planchet.
However, the plastic or paper base material can be attached to any other required object, for example, for product security. In this case, the base material is preferably produced in the form of a label. If the base material is a component part of the product to be secured, as in the case of tear-off threads, naturally any other shape is also possible. In certain applications, it may be useful to provide the luminescent substance as an invisible coating on the valuable document. It could be present over the whole surface or in the form of particular patterns, such as stripes, lines, circles or alphanumeric signs. In order to ensure the invisibility of the luminescent substance according to the invention, either a colourless luminescent substance must be used at the maximum concentration for the printing ink or coating lacquer, which is just below the level where it impairs the characteristics of the coating, or a coloured luminescent substance must be used in such a low concentration that the coating just retains its transparency.
The designation xe2x80x9cvaluable documentxe2x80x9d within the scope of the invention denotes bank notes, cheques, shares, stamps, identification papers, credit cards, passes and other documents, as well as labels, seals, packaging or other elements for product security.
The luminescent substances according to the invention have emission spectra that lie so far into the IR spectral region that they can only be detected by the detectors available for this region with great technical effort if they are added to the valuable document to be marked in the maximum quantity which does not alter the document.
Luminescent substances of this kind are designated below as xe2x80x9climit luminescent substancesxe2x80x9d.
From the safety standpoint, they have the advantage, compared with other luminescent substances that do not lie on these limits, that they are practically not employed in other technical areas and are therefore not available commercially. Furthermore, the detection technology is so complicated that the risk of analysis of the measurement parameters is relatively slight. But even if the existence of the luminescent substance was known to a forger, as already stated, he could only copy it by exactly setting all the parameters responsible for the luminescence. Luminescent substances with poorer properties either produce lasting changes to the characteristics of the valuable document, or they are no longer detectable in the detection devices.